1. Field of the Invention
The present invention relates to a method of forming a semiconductor film such as a silicon film used in a thin-film semiconductor device, integrated circuit, solar cell, charge-coupled device, or other device. The invention also relates to a low-pressure chemical vapor deposition apparatus used to form a semiconductor film and also to a thin-film semiconductor device which employs a semiconductor film and can be applied to an active-matrix liquid crystal display.
2. Description of Related Art
In recent years, liquid-crystal displays having larger viewing screens and higher resolution have been manufactured. With this trend the driving method has shifted from the simple matrix method to the active-matrix method to provide a display having a larger amount of information. The active-matrix method permits the fabrication of a liquid-crystal display having hundreds of thousands of pixels. In this liquid-crystal display, a switching transistor is provided for each pixel. Molten quartz plates and transparent insulating plates made of glass or other material are used as the front and back plates of various liquid-crystal displays. The transparent insulating plates enable the fabrication of transmission-type display devices.
However, in order to increase the area of the viewing screen or to reduce the cost, it is essential to use cheap, normal glass as insulating plates or substrates. Therefore, there is a demand for a technique capable of forming thin-film transistors operating stably on inexpensive glass plates which activate an active-matrix liquid-crystal display while maintaining economical merit.
Usually, semiconductor films such as amorphous silicon films or polycrystalline silicon films are used as the active layers of thin-film transistors. When driver circuits are integrated with thin-film transistors, the use of polysilicon is advantageous, because it offers a high operating speed.
In order to make such devices, a technique is required for fabricating a thin-film semiconductor device having a normal glass plate on which an active layer consisting of a semiconductor film such as a polysilicon film is formed. Where normal glass plates are used, a great restriction is imposed on the process. In particular, the maximum temperature must be below about 600.degree. C., i.e., below the strain point of the glass. Therefore, there is a demand for a low-temperature processing technique for fabricating thin-film transistors capable of operating a liquid-crystal display and the active layer of thin-film transistors capable of operating driver circuits at a high speed.
One conventional LPCVD (low-pressure chemical vapor deposition) method of depositing such a semiconductor film uses an LPCVD system in which the lowest attainable base pressure is 10.sup.-3 to 10.sup.-4 torr. The deposition pressure is set above approximately 40 mtorr by the system. A semiconductor film such as a silicon film is deposited by elevating temperature to 610.degree.-640.degree. C. which is equal to or slightly higher than the strain point of the glass. Another LPCVD method makes use of an LPCVD system having a reaction chamber which is evacuated at an effective pumping speed of about 1 to 3 SCCM/mtorr or less. The deposition temperature is elevated to the maximum permissible temperature of approximately 600.degree. to 620.degree. C. at which the glass plates or substrates can be used. Monosilane (SiH.sub.4) is supplied as a raw material gas at a flow rate of about 10 SCCM (Standard Cubic Centimeters per Minute). The partial pressure of the monosilane is set to about several millitort. Under this condition, a semiconductor film such as a silicon film is deposited (see Solid State Devices and Materials, 1991, Extended Abstracts, p. 614).
A further method of forming a semiconductor layer acting as an active layer is described in Japanese Patent Laid-Open No. 307776/1988. In particular, a semiconductor film such as a silicon film which will become the active layer is deposited on an insulating substrate at a temperature lower than 570.degree. C. by low-pressure CVD. Then, the substrate is thermally treated at a temperature lower than 640.degree. C. for about 24 hours to form a crystallized semiconductor film, thus improving the characteristics of the thin-film transistors.
Yet other methods use rf magnetron sputtering or plasma-enhanced CVD. In particular, an amorphous silicon film is deposited at a temperature below about 300.degree. C. by the magnetron sputtering or plasma-enhanced CVD. Then, the film is illuminated with various laser radiations to form a silicon film acting as the active layer of a thin-film transistor (Jpn. J. Appl. Phys. 28, p. 1871 (1989); and Research Report EID-88-58, Institute of Electronics and Information Communication Engineers).
These prior art techniques have various drawbacks. In the second method of thermally treating the substrate after silicon film is deposited, the thermal treatment temperature is too high to use glass plates. If this annealing temperature is lowered below about 600.degree. C., the processing time is as long as tens of hours. This also inhibits the use of glass plates. In addition, the manufacturing process is redundant compared with the first-mentioned LPCVD method. As a result, the productivity is decreased, and the price of the commercial product is increased. In the method using laser illumination after a silicon film is deposited, the semiconductor characteristics vary greatly from device to device. This makes it impossible to form a number of thin-film semiconductor devices uniformly in a large area. Additionally, the manufacturing process is more complex and more redundant than the first-mentioned LPCVD process, in the same way as the second-mentioned method. Hence, the productivity is low. Also, expensive processing equipment must be purchased. Furthermore, the cost is increased. Accordingly, various problems are associated with these processes.
On the other hand, where the prior art LPCVD is utilized to form a semiconductor film such as a silicon film, the deposition apparatus and deposition techniques are not yet advanced. Therefore, if such a semiconductor film is used as an active layer, the obtained semiconductor characteristics are not satisfactory. Hence, the produced film cannot be adequately used as switching devices or driver circuits of a high-definition liquid-crystal display of high image quality.